Dark Matter in the minimal Inverse Seesaw mechanism

TL;DR

This paper explores a minimal inverse seesaw model that can simultaneously generate neutrino masses and provide a keV sterile neutrino dark matter candidate, addressing relic density and observational constraints.

Contribution

It introduces an extension with a scalar singlet to enhance dark matter production and broadens the viable parameter space for sterile neutrino dark matter within the inverse seesaw framework.

Findings

Sterile neutrino can account for the 3.5 keV line as dark matter.

Relic density can be achieved via freeze-in decay of heavy neutrinos.

Extension with scalar singlet improves dark matter abundance in low mass region.

Abstract

We consider the possibility of simultaneously addressing the dark matter problem and neutrino mass generation in the minimal inverse seesaw realisation. The Standard Model is extended by two right-handed neutrinos and three sterile fermionic states, leading to three light active neutrino mass eigenstates, two pairs of (heavy) pseudo-Dirac mass eigenstates and one (mostly) sterile state with mass around the keV, possibly providing a dark matter candidate, and accounting for the recently observed and still unidentified monochromatic 3.5 keV line in galaxy cluster spectra. The conventional production mechanism through oscillation from active neutrinos can account only for $\sim 43\%$ of the observed relic density. This can be slightly increased to $\sim 48\%$ when including effects of entropy injection from the decay of light (with mass below 20 GeV) pseudo-Dirac neutrinos. The correct relic density can be achieved through freeze-in from the decay of heavy (above the Higgs mass) pseudo-Dirac neutrinos. This production is only effective for a limited range of masses, such that the decay occurs not too far from the electroweak phase transition. We thus propose a simple extension of the inverse seesaw framework, with an extra scalar singlet coupling to both the Higgs and the sterile neutrinos, which allows to achieve the correct dark matter abundance in a broader region of the parameter space, in particular in the low mass region for the pseudo-Dirac neutrinos.